A conventional open-loop hydraulic system includes a pump that draws low-pressure fluid from a tank, pressurizes the fluid, and makes the pressurized fluid available to multiple different actuators for use in moving the actuators. In this arrangement, a speed of each actuator can be independently controlled by selectively throttling (i.e., restricting) a flow of the pressurized fluid from the pump into each actuator. For example, to move a particular actuator at a high speed, the flow of fluid from the pump into the actuator is restricted by only a small amount. In contrast, to move the same or another actuator at a low speed, the restriction placed on the flow of fluid is increased. Although adequate for many applications, the use of fluid restriction to control actuator speed can result in flow losses that reduce an overall efficiency of a hydraulic system.
An alternative type of hydraulic system is known as a closed-loop hydraulic system. A closed-loop hydraulic system generally includes a pump connected in closed-loop fashion to a single actuator or to a pair of actuators operating in tandem. During operation, the pump draws fluid from one chamber of the actuator(s) and discharges pressurized fluid to an opposing chamber of the same actuator(s). To move the actuator(s) at a higher speed, the pump discharges fluid at a faster rate. To move the actuator with a lower speed, the pump discharges the fluid at a slower rate. A closed-loop hydraulic system is generally more efficient than a conventional hydraulic system because the speed of the actuator(s) is controlled through pump operation as opposed to fluid restriction. That is, the pump is controlled to only discharge as much fluid as is necessary to move the actuator(s) at a desired speed, and little or no throttling of a fluid flow is required.
An exemplary closed-loop hydraulic system is disclosed in U.S. Patent Publication 2008/0250785 of Griswold that published on Oct. 16, 2008 (the '785 publication). In the '785 publication, a multi-actuator hydraulic system is described that has flow combining functionality. The hydraulic system includes a first circuit having a first hydraulic actuator connected to a first pump in a closed-loop manner, and a second circuit having a second hydraulic actuator connected to a second pump in a closed manner. The hydraulic system also includes a third pump connected in an open-loop manner to the first and second circuits to provide additional flow to the first and second circuits.
Although an improvement over existing meterless hydraulic systems, the meterless hydraulic system of the '785 publication described above may still be less than optimal. In particular, because the third pump is connected to the first and second circuits in an open-loop manner, excessive losses may still be realized.
The hydraulic system of the present disclosure is directed toward solving one or more of the problems set forth above and/or other problems of the prior art.